A square can capable of securing high sealing performance even if a curvature radius of a seamed portion at a seamed corner portion is reduced, enabling a reduction in a countersink depth, reduced in size, having excellent sealing ability and storage efficiency and a method and an apparatus for double seaming the square can. A second seaming roll (55) is guided by a model cam for second seaming having a model cam surface for second seaming formed in such a shape that it is swelled from a model cam surface for first seaming outwards at a corner seamed portion (5). Accordingly, the seaming shape of the corner seamed portion (5) is formed in such a shape that the seaming width thereof at the center of the corner seamed portion is larger than the seaming width of a linear sealed portion (4) and is swelled outwards to absorb an increase in sheet thickness at the corner seamed portion. Also, a seaming wall (6) is formed in an obliquely inclined seamed shape so that the overlap of a cover hook (8) with a body hook (10) of a prescribed amount can be secured without allowing the cover hook (8) to fall from the body hook (10) so as to maintain excellent sealing ability.